Writing Macro Functions and Using the EXITM Directive

• Microsoft Macro Assembler for MS-DOS, versions 6.0, 6.0a, and 6.0b
• Microsoft Macro Assembler for OS/2, versions 6.0, 6.0a, and 6.0b

SUMMARY

Beginning with the Microsoft Macro Assembler (MASM) version 6.0, a macro can be written that will return either a text or numeric value. This is called a "Macro Function" and is implemented with the EXITM directive.

In MASM versions earlier than 6.0, the EXITM directive was used to exit a macro immediately. There were no arguments or parameters. With versions 6.0 and later, the EXITM directive takes an optional argument that specifies the return value. This argument must be either text, a text equate name, or the result of another macro function. In addition, the EXITM directive must be at the end of a macro block if it is to be used to return a value.

MORE INFORMATION

The two examples below illustrate a couple of ways to use a macro function. In the first example, a macro function is used to concatenate a text string to create unique labels for a procedure definition. The example code is followed by an extract from the list file showing the concatenated names for the procedures.

The second example illustrates how a numeric value may be returned by using the expansion operator (%). This example is followed by a preprocessor listing showing the resultant code.

Sample Code 1

; Assemble options needed: /c /Fl

        .model small

paste   MACRO   arg1,arg2       ;Macro function that concatenates
        EXITM   <arg1&&arg2>    ;  strings arg1 and arg2
ENDM

n_proc  MACRO new_name          ;Macro that calls the paste
        paste   (%prefix,&new_name) PROC ;  macro function.
        paste   (%prefix,&new_name) ENDP ; The % expansion operator
ENDM                            ;  evaluates the expression
                                ;  associated with prefix.
        .code

        n_proc   fun_1
        n_proc   fun_2

        END

Macros:

       N a m e         Type

n_proc .............   Proc
paste  .............   Func

       N a m e            Type    Value   Attr

mycode_fun_1 ..........  P Near   0000    _TEXT  Length= 0000 Public
mycode_fun_2 ..........  P Near   0000    _TEXT  Length= 0000 Public

Sample Code 2

; Assemble options needed: /c /EP

         .model small

get_rate MACRO val              ; macro function that accepts a
         LOCAL thin             ;   value and returns the results of
         thin TEXTEQU <val*300> ;   an expression.
         EXITM %thin
ENDM

         FOR b_rate, <1, 4, 8>     ; Equivalent to IRP with MASM 5.1a.
            BYTE b_rate
            WORD get_rate(b_rate)  ; Call to Macro Function.
         ENDM
ENDM

         .code

         baudrate

         END

   .model small

   .code
         BYTE 1
      ??0000 TEXTEQU <1*300>  ; An expression
         WORD 300             ; Call to Macro Function
         BYTE 4
      ??0001 TEXTEQU <4*300>  ; An expression
         WORD 1200            ; Call to Macro Function
         BYTE 8
      ??0002 TEXTEQU <8*300>  ; An expression
         WORD 2400            ; Call to Macro Function
   end